It also only awarded mod points to people who browsed the site on a regular basis but not all the time - so logging in every day or two for an hour would give you them fairly often, but if you spent all day on there you never had mod points. I don't think I got them until I started to use the site less and less which was years after signing up in 2000.
It keeps people from simultaneously expressing their opinion and downvoting other opposing opinions.